1. Field of the Invention
This invention relates to a process for producing a finely divided dispersion of a solid having an average particle size of 10 nm to 10 μm, by collision of a preliminary dispersion under high pressure, in an atmosphere of water vapor.
2. Description of the Related Art
Devices such as ball mills or agitated ball mills are available for the production of finely divided dispersions. A disadvantage of these devices is the abrasion of the grinding media, for example, materials made of glass, ceramic, metal or sand. This abrasion restricts the use of the dispersions thus produced in fields where only small amounts of impurities are tolerated, such as for example, the polishing of sensitive surfaces.
Higher energy inputs are possible using a planetary kneader/mixer. The efficiency of this system is dependent, however, on the mixture being processed having a sufficiently high viscosity in order to introduce the high shear energies necessary for the dispersion of the particles.
Very finely divided dispersions can be produced using high-pressure homogenizers wherein a preliminary dispersion under high pressure strikes the armored wall regions of a chamber. It has become apparent, however, that the chamber of such a device, despite the armor, is subject to severe wear. The division of the preliminary dispersion into two streams, which are released through a nozzle and meet one another precisely, lessens the abrasion but does not solve the problem. In particular, the nozzles are subject to a high stress and centering of the preliminary dispersions directed towards one another is difficult.
The abrasion during the production of dispersions is clearly decreased if the divided streams of the preliminary dispersion are released under high pressure onto a common collision point which is located in a gas-filled reaction chamber. The cavitation on material walls is minimized as a result of this arrangement. This is unlike the high-pressure devices described above, which operate in a reaction chamber filled with liquid. In the process, the gas flow also assumes the task of transporting the dispersion out of the reaction chamber and of cooling the dispersion.
A disadvantage of this process is the working-up of the gas-dispersion mixtures. To achieve economically rational throughputs, large quantities of gas have to be used. The separation of this gas requires increased expenditure on apparatus such as, for example, appropriately dimensioned degassers. The thermal conductivity, lowered by reason of the high gas content, requires larger dimensioned and hence costlier cooling devices for cooling of the mixture which may optionally be required. This process is particularly disadvantageous in cases where surface-active substances are added to the preliminary dispersion as dispersing agents. The introduction of gas can give rise to an undesirable foaming, which may greatly impede the working-up of the dispersion. The addition of defoaming agents is unsuitable for many dispersion applications, as these additives may have adverse effects during the application of dispersions.